Problems and Solutions of Laser Welding of Aluminum Alloys-News-Dongguan Dingshen Metal Materuak Co.,Ltd.

Problems and Solutions of Laser Welding of Aluminum Alloys

AL3003 aluminium rod 8-100mm and AL3003 aluminium plate 1-100mm for laser welding

1. High Reflectivity and Heat Conductivity of Aluminum Alloy Surface

This characteristic can be explained by the microstructure of aluminium alloy. Because of the high density of free electrons in aluminium alloy, the free electrons are forced to vibrate by laser (strong electromagnetic wave) to produce secondary electromagnetic wave, which results in strong reflection wave and weak transmission wave. Therefore, the surface of aluminium alloy has high reflectivity and low absorption rate to laser. At the same time, the Brownian motion of free electrons is stimulated and becomes more intense, so the aluminium alloy also has high thermal conductivity.

Aiming at the high reflectivity of aluminium alloy to laser, a lot of research has been done at home and abroad. The experimental results show that proper surface pretreatment such as sandblasting, sandpaper grinding, surface chemical etching, surface plating, graphite coating and oxidation in air furnace can reduce the beam reflection and effectively increase the absorption of beam energy by aluminium alloy. In addition, from the aspect of welding structure design, manual hole-making or optical collector joint, V-groove opening or tailor-welded (splicing gap equivalent to manual hole-making) can increase the laser absorption of aluminium alloy and obtain greater penetration. In addition, reasonable design of welding gap can be used to increase the absorption of laser energy on the surface of aluminium alloy.

2. Effect of keyhole and plasma on laser welding of aluminium alloy

During laser welding of aluminium alloy, the appearance of small holes can greatly improve the laser absorption rate of material, and more energy can be obtained by welding. The boiling point of aluminium element and Mg, Zn and Li in aluminium alloy is low, easy to evaporate and vapor pressure is high. Although this is helpful to the formation of small holes, the cooling effect of plasma (shielding and absorption of energy by plasma, which reduces laser irradiation on aluminium alloy) The energy input of the base metal makes the plasma itself "overheated", but it hinders the continuous existence of keyholes and easily produces welding defects such as blowhole, thus affecting the welding forming and mechanical properties of joints. Therefore, the induction and stabilization of keyholes become a key point to ensure the quality of laser welding.

Because of the high reflectivity and thermal conductivity of aluminium alloys, it is necessary for laser to have higher energy density to induce the formation of small holes. Since the energy density threshold is essentially controlled by the alloy composition, the stable welding process can be obtained by controlling the process parameters and choosing the appropriate laser power to ensure the appropriate heat input. In addition, the threshold of energy density is also affected to some extent by the types of protective gases. For example, N2 gas can easily induce small holes in laser welding of aluminium alloys, while He gas can not induce small holes. This is because exothermic reaction can take place between N2 and Al, resulting in the formation of Al-N-O ternary compounds to improve the laser absorption.

3. stomatal problems

Different kinds of aluminium alloys produce different types of pore. It is generally believed that the following types of pore are produced in the welding process of aluminium alloy.

1) Hydrogen pore. After melting in hydrogen-containing environment, the internal hydrogen content of aluminium alloy can reach more than 0.69ml/100g. However, after solidification, the hydrogen solubility in equilibrium state is only 0.036ml/100g at most, and the difference between them is nearly 20 times. Therefore, in the process of transition from liquid to solid, excess hydrogen in liquid aluminium must be precipitated. If the precipitated hydrogen does not float smoothly, bubbles will accumulate and remain in the solid aluminium alloy as pore.

2) Protect gas-generated pore. In the process of high-energy laser welding of aluminium alloy, the protective gas is involved in the molten pool to form bubbles due to the strong evaporation of the metal in front of the small hole at the bottom of the molten pool. When the bubbles can not escape and remain in the solid aluminium alloy, they become bubbles.

3) The stomata produced by the collapse of small holes. In the laser welding process, when the surface tension is greater than the vapor pressure, the small holes will not maintain stability and collapse, and the holes will be formed before the metal can be filled. There are also many practical measures to reduce or avoid pore defects in laser welding of aluminium alloys, such as adjusting laser power waveform, reducing unstable collapse of small holes, changing the focus height and inclined irradiation of beams, applying electromagnetic field effect in welding process and welding in vacuum. In recent years, the technology of filling wire or prefabricated alloy powder, composite heat source and double focus technology have been used to reduce pore formation, which has a good effect.

4. Crack problem

Aluminum alloys are typical eutectic alloys. Hot cracks are more likely to occur under rapid solidification of laser welding. Low melting point eutectics such as Al-Si or Mg-Si, which are formed at the columnar crystal boundary when the weld metal crystallizes, are the causes of cracks. In order to reduce thermal cracks, laser welding can be carried out by filling wire or pre-alloying powder. By adjusting the laser waveform, controlling the heat input can also reduce the crystallization cracks.

Adddate:2018/12/19 14:30:24 Hits： 918